The present invention relates to improvements in the reversed phase suspension polymerization process for the production of (meth)acrylic acid type highly water-absorbent resins.
While a variety of water-absorbent resins are known today, the (meth)acrylic acid type highly waterabsorbent resins obtainable by subjecting a partially neutralized salt of (meth)acrylic acid to reversed phase suspension polymerization are particularly important.
In regard to the technology of reversed phase suspension polymerization of such a partially neutralized salt of (meth)acrylic acid, a large number of patent applications have been filed to this day as listed below.
Japanese Patent Publication No. 30710/1979 (Laid-open KOKAI 53-46389): a dispersing agent defined. PA0 Japanese Laid-open Patent Application KOKAI 56-26909 (1981): polymerization under reduced pressure. PA0 Japanese Laid-open Patent Application KOKAI 56-93716 (1981): an aliphatic alcohol used concomitantly as a medium. PA0 Japanese Laid-open Patent Application KOKAI 56-131608 (1981): a dispersing agent specified. PA0 Japanese Laid-open Patent Application KOKAI 56-147806 (1981): an aliphatic ketone is used as a medium. PA0 Japanese Laid-open Patent Application KOKAI 56-161408 (1981): certain two kinds of catalysts are used. PA0 Japanese Laid-open Patent Application KOKAI 57-44627 (1982): crosslinking with a crosslinking agent. PA0 Japanese Laid-open Patent Application KOKAI 57-94011 (1982): a carboxy groups-containing polymer is used as a dispersing agent. PA0 Japanese Laid-open Patent Application KOKAI 57-98513 (1982): a basic nitrogen-containing polymer is used as a dispersing agent. PA0 Japanese Laid-open Patent Application KOKAI 57-128709 (1982): a certain anti-tack agent is used. PA0 Japanese Laid-open Patent Application KOKAI 57-158209 (1982) : a certain protective colloid is used. PA0 Japanese Laid-open Patent Application KOKAI 57-167302 (1982): a certain nonionic surfactant is used. PA0 Japanese Laid-open Patent Application KOKAI 57-192416 (1982): a certain anti-tack agent is used. PA0 Japanese Laid-open Patent Application KOKAI 57-198714 (1982): partial dehydration of a polymer slurry. PA0 Japanese Laid-open Patent Application KOKAI 58-32607 (1983): addition of a surfactant during or after polymerization. PA0 Japanese Laid-open Patent Application KOKAI 58-42602 (1983) reacting the polymer with a crosslinking agent. PA0 Japanese Laid-open Patent Application KOKAI 58-117222 (1983): reacting with a crosslinking agent. PA0 Japanese Laid-open Patent Application KOKAI 59-8711 (1984) addition of inorganic powders during or after polymerization. PA0 Japanese Laid-open Patent Application KOKAI 59-62665 (1984): crosslinking with a crosslinking agent. PA0 Japanese Laid-open Patent Application KOKAI 60-179410 (1985): a tertiary amino group-containing monomer is concomitantly used. PA0 Japanese Laid-open Patent Application KOKAI 60-186506 (1985): reaction with a crosslinking agent after polymerization. PA0 Japanese Laid-open Patent Application KOKAI 61-40309 (1986): a certain protective colloid is used PA0 Japanese Laid-open Patent Application KOKAI 61-43606 (1986): a sucrose fatty acid ester is used as a protective colloid PA0 Japanese Laid-open Patent Application KOKAI 61-53308 (1986): certain two kinds of protective colloids are used in combination. PA0 Japanese Laid-open Patent Application KOKAI 61-69812 (1986): reaction with a monoglycidyl compound. PA0 Japanese Laid-open Patent Publication KOKAI 57-158209 (1982), ditto KOKAI 58-32607 (1983), ditto KOKAI 60-179410 (1985) and ditto KOKAI 60-186506 (1985) give examples in which the monomer is fed by dripping to a hydrophobic medium.
Among the above literature,
Among the above literature, Japanese Laid-open Patent Publication KOKAI 58-32607 (1983) describes that the addition of a water-soluble or water-dispersible surfactant during or after polymerization results in an improvement in water absorption characteristics inclusive of initial water absorbency. In this prior art, an oil-soluble cellulose ester or cellulose ether is essentially used as a protective colloid and no surfactant is used as a protective colloid.
It is stated therein that the surfactant is added either "during" or "after" polymerization and that generally the addition thereof to a polymer slurry after polymerization reaction is preferred.
As to why the addition of a surfactant results in an improved water-absorbing characteristic, the inventor concerned postulates that the surfactant breaks the protective colloid covering polymer particles and imparts penetrability to the whole water-absorbent polymer.
As examples of said water-soluble or water-dispersible surfactant, the same patent literature lists nonionic surfactants with HLB values not less than 7, such as polyoxyethylene alkyl ethers, polyoxyethylene alkylphenol ethers, polyoxyethylene sorbitan fatty acid esters, polyoxyethylene fatty acid esters, polyoxy- ethylene-oxypropylene block polymers, etc.; anionic surfactants such as salts of fatty acids, alkylnaphthalenesulfonic acid, dialkyl sulfosuccinates, alkyl sulfate ester salts, higher alcohol sulfate ester salts, etc., and cationic surfactants such as alkylamine salts, alkyl quaternary ammonium salts and so on.
Any highly water-absorbent resin is required to satisfy the following requirements.
(a) It does not undergo uneven gelation on contact with water or body fluid.
(b) When brought into contact with water or body fluid, it shows a high absorption rate and a large absorption capacity.
(c) Even under stress loading, it retains a high absorption rate and a large absorption capacity for water and body fluid.
(d) It shows a high gel strength when swollen on absorption of water or body fluid.
(e) In production, it can be obtained in an appropriate particle size range (100-300 .mu.m) in good yield.
However, the (meth)acrylic acid type highly absorbent resin prepared by the usual reversed phase suspension polymerization process fails to well satisfy the requirements (b) and (e) although it meets the other requirements.
The present inventors also had a preconception, prior to this invention, that once charged in a single dose, a dispersing agent remains at substantially the same concentration level throughout polymerization reaction and accordingly made trials and errors on the supposition that a homogeneous polymer would be forthcoming coming if an ingenuity be excercised in the manner of monomer charging. The outcome was, however, a virtual failure.
Then, the inventors suspected that even if the dispersing agent fed in a single dose may not be destroyed or otherwise eliminated during polymerization, its effective concentration may undergo change in the course of polymerization reaction. The present invention is based on results of research endeavors made along the above way of thinking.
Thus, through an ingenuity exercised in respect to the conditions of reversed phase suspension polymerization, the present invention has successfully made it possible to produce a (meth)acrylic acid type highly water-absorbent resin meeting all the aforementioned requirements (a) though (e).